The exhaust purification system of an internal combustion engine described in WO2014/118890A comprises an upstream side exhaust purification catalyst provided in an exhaust passage of the internal combustion engine, a downstream side exhaust purification catalyst provided at the downstream side of the upstream side exhaust purification catalyst in the direction of flow of exhaust in the exhaust passage, a downstream side air-fuel ratio sensor provided between the upstream side exhaust purification catalyst and the downstream side exhaust purification catalyst in the exhaust passage, and a control device able to control the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst as “air-fuel ratio control”.
In the exhaust purification system described in WO2014/118890A, in the air-fuel ratio control, when the output air-fuel ratio of the downstream side air-fuel ratio sensor is a rich judged air-fuel ratio or less, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is switched to an air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, referred to as a “lean air-fuel ratio”). In addition, when the oxygen storage amount of the upstream side exhaust purification catalyst becomes a switching reference storage amount smaller than a maximum storable amount of oxygen or becomes larger, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is switched to an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, referred to as a “rich air-fuel ratio”). By executing such air-fuel ratio control, it is considered possible to keep NOX from flowing out from the upstream side exhaust purification catalyst.
By executing the above-mentioned air-fuel ratio control, NOX will never flow out from the upstream side exhaust purification catalyst, but unburned gas (HC, CO, etc.) will sometimes flow out. For this reason, unburned gas will periodically flow into the downstream side exhaust purification catalyst and the oxygen storage amount of the downstream side exhaust purification catalyst will gradually fall. On the other hand, in most internal combustion engines, the feed of fuel from a fuel injector is temporarily stopped during operation of the internal combustion engine in accordance with the engine operating state as “fuel cut control”. If such fuel cut control is executed, the oxygen storage amount of the downstream side exhaust purification catalyst will increase up to the maximum storable amount of oxygen. Therefore, if fuel cut control is periodically executed, due to the above-mentioned air-fuel ratio control, even if the oxygen storage amount of the downstream side exhaust purification catalyst falls, it will never reach close to zero.
In this regard, depending on the engine operating state, sometimes fuel cut control is not executed for a long time period. In this case, the oxygen storage amount of the downstream side exhaust purification catalyst falls and finally the unburned gas which flows out from the upstream side exhaust purification catalyst ends up being unable to be sufficiently removed at the downstream side exhaust purification catalyst. Therefore, in the exhaust purification system described in WO2014/118890A, when the oxygen storage amount of the downstream side exhaust purification catalyst becomes smaller, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is continuously or intermittently made the lean air-fuel ratio. Due to this, the oxygen storage amount of the upstream side exhaust purification catalyst reaches the maximum storable amount of oxygen and exhaust gas containing oxygen or NOX flows out from the upstream side exhaust purification catalyst. According to the exhaust purification system described in WO2014/118890A, as a result, it is considered possible to make the oxygen storage amount of the downstream side exhaust purification catalyst increase and restore the ability of the upstream side exhaust purification catalyst to purify the unburned gas.